XO-4 B1 timeout entering DCON mode

[Quozl]

A long duration DCON freeze and unfreeze test was set up, consisting of:

dev /display
: hdd-led-off  led-storage-gpio# gpio-clr  ;
: hdd-led-on  led-storage-gpio# gpio-set  ;
: yy  hdd-led-on dcon-freeze hdd-led-off dcon-unfreeze  ;
: xx  begin yy key? until key drop  ;
gvsr xx gvsb xx

On XO-4 B1 SKU292 (4b18) with Q7B08.

About 3900 seconds into the test, a scrolling error began:

Timeout entering DCON mode

Interrupting the test, diagnosis showed:

• get-msecs returned 3baf52,
• dcon-freeze was generating the error,
• dcon-unfreeze was not generating any error,
• has-dcon-ram? returned true,
• brightness keyboard keys continued to work properly,
• mode@ returned 69 (pass-through disable, backlight enable, color swizzling enable, color anti-aliasing enable),
• select /dcon dcon-unload or dcon-load had no effect on screen image, it continued to be live,
• the alternate function register for GPIO 142 was correct (1), and the GPIO direction was correct (output), and gpio-pin@ 1 and tracked dcon-unload (1) and dcon-load (0),
• a soft reset of the DCON memory controller was ineffective, (1 42 dcon! 1 ms 0 42 dcon! ),
• a system power cycle cleared the problem.

The problem has not reproduced on two other units (SKU293, SKU294).

[Quozl]

Reproducible on all three units here when I returned from lunch.

[Quozl]

"don't believe this is a new problem --- when this happens in Linux, the DCON is power cycled" -- John.

[Quozl]

• tested DCON power cycle, dcon-suspend dcon-resume did not clear problem, dcon-suspend d# 3 ms dcon-resume did clear problem.
• tested disabling the default pull-ups on DCONSTAT and DCONIRQ#, no effect.
• compared DCON register values between working and non-working states, no difference.
• reviewed kernel driver, could not find code that would power cycle DCON for this problem,
• did find a comment that "the DCON seems to get confused if we change DCONLOAD too frequently", which we are doing half the time in Open Firmware, but adding a 25 ms delay between each change does not prevent problem.

An additional observation about the symptom: the static screen shimmers for about ten seconds after the test-dcon-freeze-glitch is stopped by key press.

[Quozl]

Fixed in svn 3543, by power cycling the DCON whenever the timeout occurs. Tested over 40k cycles.

[Quozl]

Is in Q7B16.